So-called "piggy back" freight cars have been used to carry trailers. With the emphasis on the conservation of gasoline, carrying trailers over long distances has become increasingly more important.
Several problems are presented when trailers are carried by conventional freight cars. One is that the floor level of the freight car is at a set height so that the roof of the trailer being carried must also be correspondingly high. This makes it impossible for many freight cars carrying trailers to pass under many bridges and tunnels. While some attempts have been made to lower freight car floors by using smaller 28 inch wheels instead of the standard 33 inch wheel and by lowering the center plate of the trucks, these effects have generally not resulted in substantial lowering of the freight car floor.
In conventional freight cars carrying trailers, the box portion of the trailer, which may be loaded, is about seven and one-half feet above the rail. This means that the center of gravity is very high and may cause trouble during transport.
Another problem presented by many conventional freight cars is its 89 foot length. While some trailers are 40 and 42 feet in length, the recent trend has been to build trailers of 45 foot lengths. Many times, reefers extend further lengths from the front of the trailers. An 89 foot long freight car is too small to carry two trailers. Carrying a single trailer with a single 89 foot freight car is economically inefficient in many cases so as to make it unacceptable.